The goal of this proposal is to understand how positive and negative selective factors, which configure the cell-mediated immune system and are likely to differ among members of an outbred population, impact on defense mechanisms against cancer in man. The study will focus on major histocompatibility complex (MIHC) class I-restricted T cell responses in patients with malignant melanoma because: (a) there is strong evidence for the existence of at least one common tumor-specific antigen that is specifically recognized by class I-restricted T cells in melanoma; (b) tumor-specific cytotoxic T lymphocytes (CTLs) have been cloned from the tumor-infiltrating lymphocytes (TILS) of melanomas; (c) the incidence of malignant melanoma is increasing rapidly; and (d) a large clinical program exists at this institution, making patient material readily available. A related goal is to delineate the role of class I-restricted CTL responses in mediating tumor regression in melanoma patients participating in ongoing immunotherapy clinical trials. Two aspects central to this proposal are based on studies in mice which show that determinant selection by antigen-presenting cells (positive selection) and gaps in the expressed antigen receptor repertoire of T cells (negative selection imposed principally by tolerance to self antigens) are the principal forces responsible for shaping the cell-mediated immune armamentarium of the animal. The experimental strategy is based on: (a) observations that the MHC class I allele HLA-A2 is expressed at high frequency in melanoma patients and that it is a preferred restriction element for the presentation of melanoma antigen(s) to CTLs, from patients expressing that allele; (b) there appears to be limited diversity among human melanoma antigens recognized in association with HLA-A2 by tumor-specific CTL; and (c) recent studies with virus-infected murine lymphocytes have indicated that class I-associated peptides from the dominant viral antigen are surprisingly homogeneous and can be recovered in quantities sufficient for structural characterization. This investigation will use tumor cells and TILs obtained from HLA-A2+ melanoma patients at presentation and during the course of therapy to characterize tumor-specific peptide(s) associated with the A2 protein and the antigen receptor variable region genes expressed by associated TIL. The findings will establish whether differences in determinant selection and/or receptor gene expression can be detected in the outbred population of melanoma patients. Correlation of these findings with patient characteristics will indicate the extent to which tumor-specific CTL responses affect the natural history of melanoma and the role of these responses in immunotherapy-induced tumor regression.